This application is based on and incorporates herein by reference Japanese Patent Application No. 2000-278262 filed on Sep. 13, 2000 and Japanese Patent Application No. 2001-224989 filed on Jul. 25, 2001.
1. Field of the Invention
The present invention relates to a method of manufacturing a yoke of an electric motor.
2. Description of Related Art
As shown in FIGS. 12A and 12B, a previously proposed electric motor typically includes a yoke 91, a pair of magnets 92, a rotatable shaft 93, and an armature 94.
The yoke 91 is formed as a flat cylindrical container. That is, the yoke 91 has two opposite flat portions 91a in parallel with each other in a cross section perpendicular to a central axis of the yoke 91 (i.e., a cross section shown in FIG. 12B), curved portions 91b that connect both the flat portions 91a together. Each of the magnets 92 is fixed on the inside of the corresponding curved portion 91b of the yoke 91. The rotatable shaft 93 is held in the inside of the yoke 91 such that the rotatable shaft 93 can rotate. Furthermore, the armature 94 is coaxially fixed on the rotatable shaft 93 such that the surface of the armature 94 faces the magnets 92.
As shown in FIGS. 13A to 16B, for example, such a yoke 91 can be fabricated by a method in which a metal plate is drawn into a predetermined form by performing a multistage-drawing process.
In other words, as shown in FIGS. 13A and 13B, the metal plate (not shown) is drawn into a first-drawing product 95 in the shape of an elliptically cylindrical container at an initial drawing stage of the multistage-drawing process. Here, the term xe2x80x9celliptically cylindrical containerxe2x80x9d represents a cylindrical tube with a bottom in which an opening (i.e., an inner side) thereof has an elliptical cross section in a radial direction perpendicular to an axis of the product 95. In addition, such an elliptical cross section will be simply described as xe2x80x9can elliptical shapexe2x80x9d in the following description. As shown in FIGS. 14A and 14B, subsequently, the first-drawing product 95 is further drawn into a re-drawing product 96 at a middle drawing stage of the multistage-drawing process. The re-drawing product 96 has a depth greater than that of the first-drawing product 95. Also, an external periphery of the elliptical shape of the re-drawing product 96 is smaller than that of the first-drawing product 95.
Next, as shown in FIGS. 15A and 15B, the re-drawing product 96 is further drawn into a second re-drawing product 97 at a late stage of the multistage-drawing process. The second re-drawing product 97 has a depth greater than or equal to that of the re-drawing product 96. Also, an external periphery of the elliptical shape of the second re-drawing product 97 is smaller than that of the re-drawing product 96. As shown in FIGS. 16A and 16B, furthermore, the second re-drawing product 97 is drawn into the yoke 91 at a final stage of the multistage-drawing process. In this case, an arc segment 97a with a small amount of curvature of the elliptical shape of the second re-drawing product 97 is provided as the flat portion 91a while another arc segment 97b with a large amount of curvature is provided as the curved portion 91b. 
However, as described above, the conventional method of manufacturing the yoke 91 by sequentially deforming into the desired shape thereof has the following disadvantages. In each stage of the multistage-drawing process, a thickness T95a-T97a of each arc segment 95a-97a having a small amount of curvature of the elliptical shape is thinner than a thickness T95b-T97b of the corresponding arc segment 95b-97b having a large amount of curvature. Thus, a thickness T91a of the flat portion 91a of the yoke 91 is thinner than a bard thickness T91b of the curved portion 91b. 
Therefore, the thickness T91b of the curved portion 91b on which the magnet 92 is fixed is more than necessary even though it is a small effective part as a magnetic circuit, so that the amount of wasted part of the material increases. In addition, the thickness T91a is decreased even though the flat portion 91a is the most effective portion as a magnetic circuit. As a result, the amount of magnetic reluctance increases while the output of the motor (i.e., a torque performance of the motor) decreases.
The present invention addresses the above disadvantages, and it is an objective of the present invention to provide a method of manufacturing a yoke by performing a multistage-drawing process, which allows a reduction in the amount of a material waste of the yoke while contributing a high torque performance of an electric motor.
To achieve the objective of the present invention, there is provided a method of manufacturing a yoke by drawing a plate material into a final product as the yoke by performing a multistage-drawing process, where the final product as the yoke is in the shape of a cylindrical container having a couple of flat portions parallel to each other in a cross section perpendicular to a central axis of the final product and a couple of curved portions that connect the flat portions together. The method includes steps of drawing the plate material into a first-drawing product in the shape of an elliptically cylindrical container at an initial drawing stage, the first-drawing product including an elliptical cross section having a major axial length measured along a major axis of the elliptical cross section and a minor axial length measured along a minor axis of the elliptical cross section; drawing the first-drawing product obtained at the initial drawing stage into a re-drawing product in the shape of a generally circular cylindrical container at a middle drawing stage; drawing the re-drawing product obtained at the middle drawing stage into a late-drawing product in the shape of an elliptically cylindrical container at a late drawing stage, the late-drawing product including an elliptical cross section having a major axial length measured along a major axis of the elliptical cross section and a minor axial length measured along a minor axis of the elliptical cross section, the major axis and the minor axis of the elliptical cross section of the late drawing product obtained at the late drawing stage being reversed with respect to the major axis and the minor axis of the elliptical cross section of the first-drawing product obtained at the initial drawing stage, the late-drawing product having a couple of opposed smaller-curvature arc segments of a smaller curvature and a couple of opposed larger-curvature arc segments of a larger curvature; and drawing the late-drawing product obtained at the late drawing stage into the final product as the yoke at a final stage, such that the smaller-curvature arc segments of the late-drawing product are converted to the flat portions of the final product, respectively, and the larger-curvature arc segments of the late-drawing product are converted to the curved portions of the final product, respectively.
To achieve the objective of the present invention, there is also provided a method of manufacturing a yoke by drawing a plate material into a final product as the yoke by performing a multistage-drawing process, where the final product is in the shape of a cylindrical container. The method includes steps of clamping the plate material between a first presser plate and a first die and drawing the plate material into a first-drawing product using a first punch at an initial drawing stage, one of the first presser plate and the first die having two or more protrusions opposed to the other of the first presser plate and the first die, the first-drawing product being formed in the shape of an elliptically cylindrical container at the initial drawing stage, the first-drawing product including an elliptical cross section having a major axial length measured along a major axis of the elliptical cross section and a minor axial length measured along a minor axis of the elliptical cross section, at least two of the two or more protrusions being placed along the minor axis of the first-drawing product in opposed relationship to each other; and drawing the first-drawing product obtained at the initial drawing stage into the final product as the yoke at a final drawing stage, so that the final product is formed in the shape of the cylindrical container having an inner periphery in a generally circle form and an outer periphery in a generally ellipse form.
To achieve the objective of the present invention, there is also provided a method of manufacturing a yoke by drawing a plate material into a final product as the yoke by performing a multistage-drawing process, where the final product as the yoke is in the shape of a cylindrical container having a couple of flat portions parallel to each other in a cross section perpendicular to a central axis of the final product and a couple of curved portions that connect the flat portions together. The method includes steps of clamping the plate material between a first presser plate and a first die and drawing the plate material into a first-drawing product using a first punch at an initial drawing stage, one of the first presser plate and the first die having two or more protrusions opposed to the other of the first presser plate and the first die, the first-drawing product being formed in the shape of an elliptically cylindrical container at the initial drawing stage, the first-drawing product including an elliptical cross section having a major axial length measured along a major axis of the elliptical cross section and a minor axial length measured along a minor axis of the elliptical cross section, at least two of the two or more protrusions being placed along the minor axis of the first-drawing product in opposed relationship to each other; drawing the first-drawing product obtained at the initial drawing stage into a re-drawing product in the shape of a generally circular cylindrical container at a middle drawing stage; drawing the re-drawing product obtained at the middle drawing stage into a late-drawing product in the shape of an elliptically cylindrical container at a late drawing stage, the late-drawing product including an elliptical cross section having a major axial length measured along a major axis of the elliptical cross section and a minor axial length measured along a minor axis of the elliptical cross section, the major axis and the minor axis of the elliptical cross section of the late drawing product obtained at the late drawing stage being reversed with respect to the major axis and the minor axis of the elliptical cross section of the first-drawing product obtained at the initial drawing stage, the late-drawing product having a couple of opposed smaller-curvature arc segments of a smaller curvature and a couple of opposed larger-curvature arc segments of a larger curvature; and drawing the late-drawing product obtained at the late drawing stage into the final product as the yoke at a final stage, such that the smaller-curvature arc segments of the late-drawing product are converted to the flat portions of the final product, respectively, and the larger-curvature arc segments of the late-drawing product are converted to the curved portions of the final product, respectively.
To achieve the objective of the present invention, there is also provided a method of manufacturing a yoke by drawing a plate material into a final product as the yoke by performing a multistage-drawing process, where the final product is in the shape of a cylindrical container. The method includes steps of drawing the plate material into a first-drawing product in the shape of an elliptically cylindrical container at an initial drawing stage, the first-drawing product including an elliptical cross section having a major axial length measured along a major axis of the elliptical cross section and a minor axial length measured along a minor axis of the elliptical cross section; and drawing one of the first-drawing product obtained at the initial drawing stage and a subsequent product formed from the first-drawing product into the final product as the yoke at a final drawing stage, so that the final product is formed in the shape of the cylindrical container having an inner periphery in a generally circle form and an outer periphery in a generally ellipse form. At least one of the drawing steps further includes a step of drawing the plate material or any one of the products other than the final product by clamping the same between a second die and a second presser plate and then drawing the plate material or the any one of the products other than the final product by a second punch. Two or more first portions of an inner peripheral edge of the second die, which are arranged to be placed over corresponding positions that are on the minor axis of the first-drawing product, form first curve portions. Two or more second portions of the inner peripheral edge of the second die, which are arranged to be placed over corresponding positions that are on the major axis of the first-drawing product, form second curve portions. A curvature of each second curve portion is smaller than a curvature of each first curve portion.
To achieve the objective of the present invention, there is also provided a method of manufacturing a yoke by drawing a plate material into a final product as the yoke by performing a multistage-drawing process, where the final product is in the shape of a cylindrical container. The method includes steps of clamping the plate material between a first presser plate and a first die and drawing the plate material into a first-drawing product using a first punch at an initial drawing stage, one of the first presser plate and the first die having two or more protrusions opposed to the other of the first presser plate and the first die, the first-drawing product being formed in the shape of an elliptically cylindrical container at the initial drawing stage, the first-drawing product including an elliptical cross section having a major axial length measured along a major axis of the elliptical cross section and a minor axial length measured along a minor axis of the elliptical cross section, at least two of the two or more protrusions being placed along the minor axis of the first-drawing product in opposed relationship to each other; and drawing the first-drawing product obtained at the initial drawing stage into the final product as the yoke at a final drawing stage, so that the final product is formed in the shape of the cylindrical container having an inner periphery in a generally ellipse form and an outer periphery in a generally circle form.